Micro-fluidics has found increasing use in a wide variety of biomedical applications, including detection and characterization of biological entities. The devices used for such applications are broadly referred to as “biochips”. The term biochip has been used in various contexts, but can be generally defined as a micro or nano-fabricated device that is used for processing (e.g., delivery and analysis) of biological entities (e.g., molecules, cells, etc.). This invention relates to the processing of biological entities in the form of cells. As used herein, the term cell broadly refers to any microscopic organism, including bacteria, spores, molds, yeast, plant cells, and animal cells. The invention will be primarily disclosed through the example of bacteria detection. However, it should be appreciated that the invention is generally applicable to the detection of any type of cell.
Biochips based on the impedimetric detection of biological binding events or the amperometric detection of enzymatic reactions exist. Impedimetric detection works by measuring impedance changes produced by the binding of target molecules to receptors (antibodies, for example) immobilized on the surface of microelectrodes. Amperometric devices measure the current generated by electrochemical reactions at the surface of microelectrodes, which are commonly coated with enzymes. Both of these methods can be very sensitive, but preparation of the surfaces of the electrodes (immobilization of antibodies or enzymes) is a complex and sometimes unreliable process, that can be prone to drift and tends to be very sensitive to noise produced by the multitude of species present in real samples (bodily fluids, food, soil, etc.).
A specific example of use of biochips is for the detection of live bacteria and cells from a sample. The very important requirement for the micro-fabricated, impedance-based detection system for this application is the ability to concentrate the small numbers of cells present in the sample being analyzed into the micro-fabricated volume where detection is performed. One prior art approach is to use dielectrophoresis (DEP) to capture immunobeads (microscopic beads coated with charged molecules or antibodies) carrying the cells of interest inside the detection chamber. There are two reasons to use beads. First, the dielectrophoretic force is higher in magnitude on beads in the growth media when compared to the force on cells in the media. Second, the beads could also be used for specific capture of cells.
A key shortcoming associated with existing techniques using biochips for the detection of cells and their growth is that the filtering steps and growth detection steps are separate operations performed on different devices. Various processing operations are currently required to bridge these different operations. For example, a filtering operation performed on the original sample volume of up to a half-liter may use a filter membrane to capture a sample. The filter membrane is then manually moved to a growth area to grow the cells trapped on the membrane. Thus, current filter isolation and transport operations are time consuming and are prone to a variety of errors. In addition, prior art approaches cannot be integrated in an automated way in manufacturing processes where testing of various fluids is performed. It would be highly desirable to eliminate these problems through tightly coupled filtering and cell growth detection operations.
Current methods of bacteria detection almost always involve a growth step wherein the microorganisms are cultured to increase their numbers by several orders of magnitude. Depending on the type of bacteria, this amplification by means of extended growth makes conventional detection methods extremely lengthy, taking anywhere from 2 to 7 days. It would be highly desirable to significantly reduce this amplification stage processing time.
In sum, it would be highly desirable to reduce the amount of time required for cell amplification. Finally, it would be highly desirable to simplify fluidic processing through integrated filtering and cell growth operations.